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I have issue with cellular modem when I change MTU setting for mobile network interface.

Setup:

  • debian 11
  • default MTU size from mobile carrier 1430
  • require MTU size for northbound connection 1300

Behaviour: after changing MTU size, device working properly few minutes and after lose northbound connection

MTU change for the same hardware with the same mobile carrier worked well on debian 9

Thanks in advance.

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  • How exactly is the mobile interface configured? Are you using NetworkManager, or /etc/network/interfaces and a plain DHCP client, or is PPP involved? Which DHCP client? Are you configuring the MTU size in NetworkManager/DHCP client configuration, or just on the command line? If the latter, it would get overridden if/when the DHCP lease is updated, or if the PPP session gets renegotiated. Have you (or the carrier) blocked ICMP error packets, causing the automatic Path MTU Discovery mechanism (PMTUD) to break down? Commented Jan 10 at 8:19

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